1. Field of the Invention
The present invention relates to an electrophotographic toner useful for developing an electrostatic latent image formed by electrophotography, electrostatic recording methods, electrostatic printing methods and the like methods, and electrophotographic image forming method and apparatus using the toner.
2. Discussion of the Background
Various electrophotographic image forming methods have been disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications Nos. 49-23910 and 43-24748. The methods typically include the following processes:
(1) the surface of an image bearing member (a photoreceptor) is charged (charging process);
(2) the image bearing member is exposed to light to form an electrostatic latent image thereon (latent image forming process);
(3) the latent image is developed with an electrophotographic toner to form a toner image on the image bearing member (developing process);
(4) the toner image is transferred onto a receiving material (transferring process); and
(5) the toner image on the receiving material is fixed upon application of heat, pressure, solvent vapor, or combination thereof to prepare a copy image (fixing process).
The method for developing electrostatic latent images is broadly classified into the following methods:
(1) wet developing methods using a liquid developer including a fine color pigment or dye dispersed in a liquid: and
(2) dry developing methods such as cascade methods, magnetic brush methods and powder cloud methods, which use a developer (toner) including a colorant such as carbon black dispersed in a resin.
Recently the dry developing methods are widely used.
As the method for fixing toner images, methods using a heat roller are widely used because of having good energy efficiency. Recently, the heat energy used for fixing toner images becomes smaller and smaller because of performing high speed reproduction and saving energy. In addition, it is needed to decrease the warming-up time, i.e., the time from a waiting state of an image forming apparatus for copying to a working state of the apparatus in which a copy can be reproduced, while the electric power consumption of the apparatus is minimized in the waiting state to protect environment. In the DSM (Demand-side Management) program of IEA (International Energy Agency), requirements for next generation image forming apparatuses are described. It is described in the requirements that the warming-up time should not be greater than 10 seconds and the power consumption in a waiting state, which changes depending on the copying speed, should not be greater than 10-30 watt in a copier having a copy speed not less than 30 cpm (copies per minutes). In order to meet these requirements, the power consumption of copiers must be dramatically reduced.
In attempting to meet the requirements, fixing devices using a fixing element such as heat rollers, which has a low thermal capacity and which has a quick temperature rising time, are researched. However, the devices do not bring about the desirable effects.
Therefore, it is essential for satisfying the requirements to develop a toner which can be fixed at a low temperature, to decrease the fixing temperature of image forming apparatus. In order to meet the above-mentioned requirements, the image forming apparatus must decrease the fixing temperature by about 20xc2x0 C. Therefore, a need exists for a toner having a fixing temperature lower than that of the conventional toners by 20xc2x0 C. This is a hard problem, and cannot be solved by conventional techniques. This is because when a low temperature fixability is imparted to a toner, the fixable temperature range and preservation property of the toner deteriorate.
For example, Japanese Laid-Open Patent Publications Nos. 60-90344 and 3-229264 have disclosed toners having a low fixing temperature which include a resin or a wax having a low softening point. However, such toners tend to be blocked (i.e., a so-called blocking problem occurs) in a developing device in a copier, whose temperature is increased by the heat of a fixing device. In addition, such toners tend to be blocked when preserved under high temperature conditions, for example, when preserved in a warehouse without an air conditioner in a summer season. Further, such toners have a narrow fixing temperature range (i.e., such toners easily cause a hot offset problem)
Therefore, a toner exhibiting a combination of low temperature fixability and good preservation property has not yet be developed.
Recently, demands for high quality copies increase more and more. Images having good image qualities such as high resolution cannot be obtained by a conventional toner having a volume average particle diameter of from 10 to 15 xcexcm. Therefore, a need exists for a toner having a relatively small average particle diameter. When a toner having a relatively small average particle diameter is used, various problems tend to occur. For example, an offset problem tends to occur in half tone images because the quantity of heat applied to the half tone toner images is extremely decreased when the toner particles of the toner image are present in a recess of a receiving paper.
Because of these reasons, a need exists for a toner having a combination of low temperature fixability, hot offset resistance and good preservation property.
Accordingly, an object of the present invention is to provide an electrophotographic toner having good low temperature fixability.
Another object of the present invention is to provide a toner having good hot offset resistance and good heat preservation property.
Yet another object of the present invention is to provide an image forming method and apparatus which can produce a copy upon application of relatively low heat energy without causing problems such as an offset problem.
Briefly these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained by an electrophotographic toner including at least a binder resin, a colorant and a release agent, wherein the binder resin includes one or more components which are soluble in tetrahydrofuran, wherein the one or more components have a molecular weight distribution, which is measured by a GPC (gel permeation chromatography) method, such that at least one peak is present in a range of from 1,000 to 10,000, and wherein the peak has a half width not greater than 15,000, and preferably not greater than 10,000.
The binder resin preferably includes a polyester resin. In addition, the binder resin includes one or more chloroform-soluble components and one or more chloroform-insoluble components, wherein the amount of the chloroform-soluble components is greater than that of the chloroform-insoluble components.
The binder resin preferably includes two kinds of polyester resins, wherein the first resin has a relatively low molecular weight and the second polyester resin has a relatively high molecular weight, and wherein the second polyester resin is present in the first polyester resin like islands in a sea. Namely, the second resin forms one or more domains in the first resin. The softening point of the first polyester resin is preferably 25xc2x0 C. or more lower than that of the second polyester resin.
In another aspect of the present invention, an electrophotographic image forming apparatus is provided which includes a toner bottle containing the toner mentioned above therein.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawing.